In general, a battery outer label (hereinafter simply referred to as “an outer label”) includes a heat-shrinkable resin film as a base material, a printing layer formed on a front surface of the resin film (a surface visible after the assembly) for providing thereon suitable patterns, letters, colors, etc., and a protective layer made of wear-resistant varnish or transparent lacquer for protecting the printing layer.
On a back surface of the heat-shrinkable resin film (a surface in contact with the battery), a vapor-deposited metal layer, such as of aluminum, is formed to give a metallic sheen and high luminance to the printing layer, and an adhesive layer is formed for adhesion between the label and the battery. Polyvinyl chloride is used as a material for the heat-shrinkable resin film in most cases, and polyethylene, polystyrene, polypropylene, polyethylene terephthalate, etc., are used in some cases.
The outer label is wrapped around an outer circumferential surface of a battery case (mainly made of a nickel-plated steel sheet) sealed with battery components contained therein, and is adhered thereto by heat-shrinking the entire outer label by hot air.
As to the batteries provided with the outer label including the vapor-deposited metal layer, various studies on the structure of the outer label have been made for the purpose of retaining the appearance of the battery during storage at high temperature and high humidity. For example, Patent Document 1 describes a technique of providing a heat-resistant, electrically insulating layer between the vapor-deposited metal layer and the adhesive layer of the outer label so as to avoid defects, i.e., leaching of the vapor-deposited metal layer and loss of the metallic sheen during storage at high temperature and high humidity.
Patent Document 2 describes a technique of providing a hydrophobic coating at the end faces of the outer label so as to suppress the leaching of the vapor-deposited metal layer.
Further, Patent Documents 2 and 3 also describe a technique of suppressing the leaching of the vapor-deposited metal layer by setting a potential difference between the vapor-deposited metal layer and the attachment surface (the battery surface) to be electrochemically equal or small so as to prevent or delay the formation of local cell.    [Patent Document 1] Published Unexamined Japanese Patent Publication No. H09-237614    [Patent Document 2] Published Unexamined Japanese Patent Publication No. 2002-208381    [Patent Document 3] Published Unexamined Japanese Patent Publication No. H09-306440